The present invention refers to a movable structural component for a thermomechanically stressed assembly made from a fiber reinforced ceramic, particularly for reflyable aerodynes in the aviation and aerospace technique as well as a process for producing the structural component.
Reflyable spacecrafts, like for example the Shuttle Orbiter, require for reentering into the atmosphere a protective shield which is, among other things, heat resistant. The Shuttle Orbiter of the United States of America has for this reason body and control surfaces consisting of metallic material which are covered with tiles from a reinforced fiber isolation. These tiles avoid the consequence that under the influence of the highly heated air, converted to a plasma status by the high air speed, the metallic structural elements become so highly heated as to lose their strength and shape stability and will even be destroyed under the load of the flight. Similar or identical problems result with any thermally highly stressed structural elements to be used in the tool making and engineering industry.
Assemblies, structural elements or structural parts, which are based on subsequently disposed or glued-on isolations exhibit considerable disadvantages as is known in the art.
For example the most highly loaded components of a reflyable spacecraft, which, in particular, are the control flaps and such control surfaces, must be made of extremely temperature resistant metallic alloys, so-called superalloys. These have a high specific gravity. Additionally, there is the weight of the thermal isolation. Very dense isolating materials have to be used, to have sufficient resistance against the influences onto such fairings.
Despite the use of very dense fiber isolations known from the state of the art, a heat shield such as that of the Shuttle Orbiter, requires high repair and replacement work since the deposition by gluing and the low strength of the isolating material often results in damage or even complete destruction under the described application conditions. Additionally, there is the weight of the thermal isolation which affects the total weight of the aerodyne.
It is a primary object of the present invention to overcome the mentioned technical and economical problems by providing structural components which allow material and structural elements to inherently have an overall increase of thermal and mechanical loading capacity. In connection with the construction of aerodynes, a considerable reduction in weight of the structural components and the reusability or reflyability thereof is envisaged.
The central aspect of the invention is to construct structural components, particularly for reflyable aerospacecrafts, from a fiber composite ceramic. Thereby, depending on the mechanical and thermal requirements to be addressed to individual elements of the structural components, differently produced materials, so called CMC-materials (Ceramic Matrix Composites), could be used.
The present invention thus provides a movable structural component for a thermomechanically stressed structure, which at least partially is built from a fiber reinforced ceramic. Thereby, the movable structural component comprises at least one structural element formed by a polymer infiltration and pyrolysis process (subsequently referred to as LPI-process) and at least one structural element formed by a gaseous phase infiltration or chemical vapor infiltration process (subsequently referred to as CVI-process).
With the structural components according to the invention, a 40% weight saving as well as a significant reduction of the maintenance costs compared with the state of the art is possible. The reduction of the maintenance costs results in that the structural components according to the invention are mechanically and thermally extremely loadable and thus for example during the entry of the atmosphere are less damaged or destroyed.
In accordance with the invention, fiber reinforced ceramics are considered for use which are based on high temperature resistant fibers. These are, particularly, carbon fibers imbedded within a matrix of silicon carbide (C/SiC ceramic), silicon carbide fibers imbedded within a matrix of silicon carbide (SiC/SiC ceramic) or silicon nitride (SiC/Si3N4-ceramic), aluminum oxide fibers imbedded within an matrix of aluminum oxide (Al2O3/Al2O3-ceramic), mullite fibers imbedded within a mullite ceramic or polyborosilazane fibers (SiBNC) imbedded within a polycarbosilane, polysilazane or silicon carbide matrix. The properties of these ceramic materials reinforced with filaments are mainly known and, for example, described in A. Mxc3xchlratzer and H. Kxc3x6berle in Metall (1991), page 435 cf. These materials, however, may be essentially influenced by the manner of their production and processing, respectively. A discussion of suitable fiber or ceramic materials, respectively, may be also found in xe2x80x9cAdvanced Materials 2 (1990), no. 9, pages 398-404 and xe2x80x9cJournal of European Ceramic Society 12 (1990), pages 27-41xe2x80x9d.
In accordance with the present invention, carbon reinforced silicon carbide ceramics (C/SiC ceramics), in particular, are envisaged, which, adapted to the final form of the structural element, are formed either via chemical vapor infiltration (CVI-process) or via (liquid) polymer infiltration and pyrolysis (LPI-process). The material producible according to the CVI-process is particularly suitable for mechanically highly stressed parts. In case of, for example, a control flap of an aerodyne, these are the longitudinal and transversal load bearing implements, the connecting or push-rod, the bearings and the hinges as will be further described in detail. For mechanically or thermally less stressed structural elements, a material produced according to the LPI-process is also suitable.
In accordance with an aspect of the invention, the structural component is characterized in that the at least one structural element formed by the liquid polymer infiltration and pyrolysis process is embodied as the base of the movable structural component. These mostly large sized or volumed bases are, in particular, mechanically less stressed so that they may be produced by the LPI-process.
According to another aspect of the invention, the base is a box-type segment with a bottom wall and side walls integrally formed thereon. This measure allows for a wide variation in the final size of the structural component and an accommodation to the individual purpose of use. Via the integrally formed side walls, individual box-type segments may be coupled to larger structural elements or components, respectively.
In a suitable embodiment of the invention, the bottom wall, of the at least one box-type segment, is an essentially plane surface opposite to the side walls. This embodiment avoids the formation of so-called hot spots and along with it the premature wear of the structural element by thermal and/also or mechanical load.
Further, it is within the scope of the invention that the junction region between the bottom wall and the side walls is chamfered. Also, this embodiment avoids or minimizes the formation of hot spots.
According to another aspect of the invention, the at least one box-type segment of the base is stiffened by reinforcement ribs which are, in particular, integrally disposed on the bottom wall and the side walls. These reinforcement ribs or the like avoid torsions of the structural element or the structural component, respectively, under mechanical stress and allow, in particular, a lightweight construction required for the structural elements or structural components, respectively, like, for example, the control flaps of a reflyable aerodyne. These reinforcement ribs may be arranged transversally, longitudinally or diagonally.
Furthermore, it is within the scope of the invention that the at least one box-type segment of the base has a cover or the like, which is reversibly mountable on the side walls, thereby promoting the stiffening of the segment and which, by its essentially plane surface, allocates a mechanical stress over all the structural element.
In a further and most particularly preferred embodiment of the present invention, the base is composed of several box-type segments which, as already mentioned, are connectable with each other by respective adjacent side walls. This allows the exchange of possibly damaged or destroyed individual segments and, in the construction, a great variability with respect to the size of the structural elements or structural components to be assembled.
Of high importance for a structural element or such a structural component, embodied according to the invention which, for example, is used as control flap for a reflyable aerodyne, several box-type segments are arranged side by side such that the joints between the adjacent side walls extend in a direction which is essentially parallel to a possible movement of the thermomechanically stressed assembly. This special arrangement of the box-type segments in connection with aerodynes is not only aerodynamically favorable but also avoids the formation of the so-called hot spots which may result in the destruction of the structural element or of an individual segment thereof.
As mentioned above, it is within the scope of the invention that the at least one structural element formed by a chemical vapor infiltration process is embodied as a load transmission and/or bearing element of the movable structural component. The ceramics which are produced close to their final shape by the chemical vapor infiltration process due to the increased density have a very pure matrix with fine crystalline, dense microstructure imparting a high thermomechanical resistance, stiffness, compressive strength and wear resistance to the material. Also the high fracture toughness of the so produced materials has to be emphasized.
Further in accordance with the invention, the load transmission and/or also the bearing element(s) are mounted on the base of the movable structural component for its movement.
The load transmission and bearing element comprise at least one longitudinal beam and at least one, preferably two, transversal beams which are releasably mountable to each other and on the side wall of the at least one box-type segment of the base. If there is only one box-type segment, the longitudinal and/or at least the transversal beam(s), each extend centrally between the respective side walls.
In general the at least one longitudinal beam and the at least one, preferably two, transversal beams, are about centrally received by and arranged on the base. A highest possible stability of the structural component results from this arrangement with respect to its movement relative to an assembly with which it is connected.
The load transmission and bearing element comprises at least one rod or the like which transfers a force produced by a motor over the at least longitudinal beam and the at least one, preferably two, transversal beams of the base of the movable structural component.
Preferably, the load transmission and bearing element further comprise a bearing between the at least one beam and the at least one, preferably two, transversal beams as well as the at least one rod, which is semi-spherical, spherical, dome or the like shaped. During the entrance into the atmosphere a reflyable aerodyne, particularly its control flaps, is exposed not only to thermal but also to great mechanical stresses which may cause lateral torsions and often even local press compactions at the structural component or the assembly, respectively. Such lateral torsions can be received or balanced by the bearings used according to the invention which are formed semi-spherical, spherical, dome or the like shaped.
The bearing therefore has preferably a semi-spherical, spherical, dome or the like shaped formed bearing shell which is supported by a bearing pin or bolt or the like on the at least one transversal beam and a means for receiving the bearing shell cooperating therewith which is disposed at the end side of the rod and vice versa.
For stabilization or more stable movable connection of the structural component with the assembly there is arranged preferably at least one, preferably two further bearing elements for movably connecting the movable structural component with the assembly on at least one of the side walls of the at least one box-type segment, which is/are formed particularly like a hinge.
In a particularly suitable embodiment of the structural component of the present invention, at least one, preferably two of the side walls are elongated and provided with bores at its end side for receiving corresponding bearing pins or the like of the assembly, the boring axes of which are aligned to each other or to the rotation axis of the one bearing. The boring axes thus are arranged parallel to each other or to the rotation axis of the one bearing.
In a further preferred embodiment of the structural component of the present invention, the side walls and/or longitudinal beams and/or transversal beams and/or rods are hollow sectioned. This measure promotes the desired light construction of the overall assembly having simultaneously a high stiffness.
An essential feature of the present structural component is that, for increasing its mechanical and thermal stability, coupling elements like screws, pins, rivets and the like, formed by chemical vapor infiltration are provided for a detachable connection of the structural elements with each other which particularly are used in the region of the side walls and the base. Such connecting elements naturally are exposed to high mechanical stresses and represent starting points for the formation of hot spots. The choice of material and arrangement of the connection elements contributes to the further stabilization of the structural components.
A further feature of the present invention is that the structural element for the protection and stabilization of its outer surface, particularly for protection against oxidation, is provided with a suitable protective layer, if necessary. For structural elements produced according to the CVI-process then at least one layer of about 100 xcexcm thickness is used which is formed essentially of the same material as the matrix forming material. This layer is applied by chemical vapor deposition. In the case of a C/Si ceramic, a SiC/SiC ceramic or related ceramic such a protective layer is particularly effective if at least one boron containing silicon carbide layer is provided. Parts without joining surfaces also may be equipped with a pure silicon carbide layer onto which a multiphase cover layer consisting of a glass matrix with imbedded refractory phases is applied according to German patents P 40 34 001 and P 44 43 789.
As mentioned for several times, the aforedescribed structural component for a thermomechanically stressed assembly may be embodied as a movable control flap or the like of an aerodyne. Particularly, reflyable spacecrafts which are exposed to high thermal and mechanical stresses during reentrance into the atmosphere as well as control surfaces of diverse military missiles which receive similar stresses may exploit the structural component according to the invention. It may be also envisaged to use the structural components according to the invention for producing thermally and mechanically highly stressed tools and machine parts.
The invention also refers to a process for producing the aforedescribed structural components from a fiber reinforced ceramic for a thermomechanically stressed assembly. At least one structural element is produced by the LPI-process, at least one structural element is formed by the CVI-process and the structural elements are combined to the claimed structural component in a suitable way.
The structural elements preferably are joined together by connecting elements, like screws, pins, bolts, rivets and the like produced by the CVI-process.
The afore-mentioned processes have different advantages which can be used in a suitable manner for the individual structural components, particularly the control flaps for reflyable spacecrafts.
In principle, for lightweight constructions the integral construction is to be preferred. The known manufacturing processes for fiber composite ceramics, however, allow this way of construction only in a limited range for large sized or volumed components. According to the invention, therefore, a so-called hybrid or composite construction was developed in which mechanically highly stressed structural elements of a structural component, for example these are with the afore mentioned control flap the longitudinal and transversal beams (load bearing implements), the rod, the bearings and the hinges, are produced by the so-called CVI-process, particularly the gradient CVI-process. This process provides a high performance material with respect to its thermomechanical properties. The essential feature of these materials is their matrix with fine crystalline, dense structure which imparts the high thermomechanical resistance, stiffness, pressure resistance and wear resistance to the material. Essential is also the high fracture toughness of this material in connection with the construction of security structural members.
Structural elements which are less mechanically stressed, like the control flap body of a reflyable spacecraft according to the invention are produced according to the so-called LPI process. Generally either fabric cuts are disposed on molds by means of the wet laminating process or disposed dry in a forging die and filled with the matrix forming resin according to the RTM (Resin Transfer Molding) process.
According to the invention structural elements formed by the LPI-process are produced as follows: Fabric cuts from thermally highly stressable fibers are disposed on positive molds having a shape close to the end shape or into such forging dies; the disposed fabric cuts preferably are impregnated with an organic polymer or polymer resin corresponding to the fiber; then the material is cured under increased temperature and pressure and the so formed green compact is submitted to a pyrolysis treatment at about 900xc2x0 to 1,600xc2x0 C. for producing a fiber reinforced matrix or the desired ceramic material, respectively.
In a preferred way the curing takes place at 200xc2x0 C. and at about 5 bar and the pyrolysis treatment at about 1,200xc2x0 C.
To protect the aforeproduced structural elements against oxidation, if necessary,xe2x80x94this is particularly required in case carbon fibers or fibers covered with pyrocarbon are used for producing the ceramicxe2x80x94, according to the invention, they are provided at least partly with at least one protective layer produced by chemical vapor deposition (CVD process). The material forming the protective layer preferably will correspond to the material forming the matrix.
For producing the at least one structural element produced by chemical vapor infiltration (CVI process), a fabric layer of thermally highly stressable fibers is at least partly and spaced apart, provided with a suitable adhesive, the fabric layer then wound up to an essentially tube shaped fiber preform with suitable diameter, the fiber preform inserted into a chemical vapor infiltration reactor and submitted to a gradient chemical vapor infiltration under the action of a methyltrichlorsilane/hydrogen process gas or an equivalent process gas. Details of this process can be derived from the following example.
Essentially a temperature gradient of between 700xc2x0 C. inside the tube shaped fiber preform and of about 1,150xc2x0 C. in the reactor is adjusted.
The process according to the invention is characterized in that thermally highly stressable fibers are incorporated in a respective matrix bed which are chosen from the group of carbon, silicon carbide, aluminum oxide, mullit and/or polyborosilazane fibers.
For forming the matrix, starting materials will be chosen, which essentially correspond to the fiber material. Preferably a C/SiC ceramic is used.